Many lubricating oil compositions have been proposed hitherto. For example, JP2011236407 discloses a Fischer-Tropsch derived base oil (FT oil) which has a high viscosity index and has the merit of reducing the amount of viscosity index improver used. JP2009520078 discloses a lubricating agent obtained by mixing a low-viscosity FT oil with a high viscosity Group 1 oil (solvent refined mineral oil). Further, JP2012193255 discloses a gear oil obtained by mixing a low-viscosity mineral oil-based highly refined oil with a high-viscosity solvent refined mineral oil.
However, the actual situation is that, if it is car transmissions that are taken into consideration as the application, there are no lubricating oil compositions existing in the prior art, which improve fuel economy as required in said application, which have load-resisting properties, and which satisfy all the oil film retention properties and low temperature viscosity characteristics. In order to prevent fatigue damage such as the pitting caused on gear-teeth surfaces, it is important in particular to improve the oil film retention properties. At the same time, in order to improve the load-resisting capability of gear oils, it is necessary to use chemically active additives, but then there is the problem that they cause metal corrosion.
The object of the present invention is therefore to offer an automotive transmission (especially a fuel-saving type) which satisfies all requirements as regards the properties of resistance to churning, maintenance of the oil film and low-temperature viscosity.